This invention relates to printing plates which can be made without using a negative. More specifically, it relates to a laser-imageable printing plate. Such plates are particularly useful for flexographic printing, but can be used for offset and lithographic printing.
Flexography is a method of printing that is commonly used for high-volume runs. Flexography is employed for printing on a variety of substrates such as paper, paperboard stock, corrugated board, films, foils and laminates. Newspapers and grocery bags are prominent examples. Coarse surfaces and stretch films can be economically printed only by means of flexography. Flexographic printing plates are relief plates with image elements raised above open areas. One type of flexographic printing plate resembles a transparent or translucent plastic doormat when it is ready for use. The plate is somewhat soft, and flexible enough to wrap around a printing cylinder, and durable enough to print over a million copies.
Such plates offer a number of advantages to the printer, based chiefly on their durability and the ease with which they can be made. Further improvements, to the degree of resolution (fineness of detail) which can be obtained as well as reductions in cost, would expand the usefulness of these plates. The present invention allows both increased resolution by use of laser processing, and reductions in cost through the elimination of the use of a negative to make the printing plate.
A typical flexographic printing plate as delivered by its manufacturer is a multilayered article made of, in order, a backing, or support layer; one or more unexposed photocurable layers; a protective layer or slip film; and a cover sheet. The backing layer lends support to the plate. It is typically a plastic film or sheet about 5 mils or so thick, which may be transparent or opaque. Polyester films, such as polyethylene terephthalate film, are examples of materials that can be suitably used as the backing. When only a single photocurable layer is present, it may be anywhere from about 25-275 mils thick, and can be formulated from any of a wide variety of known photopolymers, initiators, reactive diluents, fillers, etc. In some plates, there is a second photocurable layer (referred to as an xe2x80x9covercoatxe2x80x9d or xe2x80x9cprintingxe2x80x9d layer) atop this first, base layer of photocurable material. This second layer usually has a similar composition to the first layer, but is generally much thinner, being on the order of less than 10 mils thick. The slip film is a thin (about 0.1-1.0 mils) sheet which is transparent to UV light that protects the photopolymer from dust and increases its ease of handling. The cover sheet is a heavy, protective layer, typically polyester, plastic or paper.
In normal use, the printer will peel the cover sheet off the printing plate, and place a negative on top of the slip film. The plate and negative will then be subjected to flood-exposure by UV light through the negative. The areas exposed to the light cure, or harden, and the unexposed areas are removed (developed). Typical methods of development include washing with various solvents or water, often with a brush. Other possibilities for development include use of an air knife or heat plus a blotter.
Exposure of the printing plate is usually carried out by application of a vacuum to ensure good contact between the negative and the plate. Any air gap will cause deterioration of the image. Similarly, any foreign material, such as dirt and dust between the negative and the plate results in loss of image quality.
Even though the slip films are thin and made from transparent materials, they still cause some light scattering and can somewhat limit the resolution which can be obtained from a given image. If the slip film were eliminated, finer and more intricate images could be obtained.
Finer resolution would be particularly desirable for the reproduction of elaborate writing as in the case of Japanese characters, and for photographic images.
A negative can be a costly expense item. For one thing, any negative which is used for printing must be perfect. Any minor flaw will be carried through onto each printed item. As a consequence, effort must be expended to ensure that the negative is precisely made. In addition, the negative is usually made with silver halide compounds which are costly and which are also the source of environmental concerns upon disposal.
Given these considerations, it is clear that any process which would eliminate the use of the negative, or reduce the light scattering effects and other exposure limitations of the slip films, would yield significant advantages in terms of cost, environmental impact, convenience, and image quality over the present methods.
The inventors have found a way to obtain these advantages by using a laser that is guided by an image stored in an electronic data file to create an in situ negative on a modified slip film, and then exposing and developing the printing plate in the usual manner. As a result, the printer need not rely on the use of negatives and all their supporting equipment, and can rely instead on a scanned and stored image. Such images can be readily altered for different purposes, thus adding to the printer""s convenience and flexibility. In addition, this method is compatible with the current developing and printing equipment, so expensive alterations to the other equipment are not required.
Laser engraving of various materials, such as wood and metal, is well known. Laser engraving of cured hard rubber or lithographic plates is also known. If this procedure were applied to a flexographic printing plate, the plate would first be exposed to UV light without an image. Then the laser would be used to engrave an image on the hardened plate. This has been attempted, but found to be too slow to be commercially competitive. Flexographic printing plates require a high relief (generally, 30-40 mil high letters) which take a long time to engrave.
Direct exposure of a photopolymer using a laser is also known. This procedure uses a precisely guided laser to replace the UV flood lamps which are normally used to expose the plate. U.S. Pat. No. 4,248,959, issued to Jeffers et al. Feb. 3, 1981, relates to the direct exposure of a photosensitive polymer plate using a laser guided by a computer-generated image. The patentees disclose using lasers that emit in the region above 450 nm, but that lasers that emit below 450 nm may also be usable. Such lasers include the helium-neon (spectral lines at 632.8, 1150 and 3390 nm), the argon ion (emitting at specific lines between 333.7 and 528.7 nm), the krypton (emitting between 337 and 858 nm), and the CO2 (emits at 1060 nm). The disclosed method is not suitable for the development of flexographic printing plates, again because the thickness of the plate hampers the cure. Again, the process is too slow to be commercially competitive.
Other efforts have focused on generating an image directly in contact with a photocurable layer. U.S. Pat. No. 5,015,553 issued to Grandmont et al. May 14, 1991 relates to a method of making a UV photoresist for a printed circuit board, using a computer-assisted design (CAD) driven photoplotter which selectively exposes a photographic imaging layer without affecting the underlying UV sensitive photoresist. The image layer is then chemically developed on the board and used as an in situ mask for the underlying UV resist during exposure to UV light. After the exposure, the image layer is peeled off to allow conventional processing of the resist. The process requires at least two development steps for the entire plate, and also requires the use of a peelable cover sheet interposed between the image layer and the photocurable layer. The patentee discloses an epoxy fiberglass substrate that carries a uniform layer of metal, e.g., copper. A photoresist layer sensitive to UV light is superimposed on the copper layer. A thin sheet of strip base photographic film overlays the photoresist. The structure is then mounted on a moving table of a plotter and exposed by a beam of light that moves relative to the film. Once exposed, the film is developed. After the image is fixed in the film, the composite structure is exposed to a UV flood lamp. Darkened areas in the image of the film absorb the ultraviolet light and shield the underlying resist. Where the film remains transparent, the ultraviolet light passes to the resist layer and polymerizes the underlying portions.
Laser ablation of polymers from relatively insensitive substrates is known. U.S. Pat. No. 4,020,762 issued to Peterson May 3, 1977 relates to a method of making a sensitized aluminum printing plate for offset lithography. An aluminum sheet was coated with a mixture of finely divided carbon, nitrocellulose, a non-oxidizing alkyd resin, a diazo sensitizer, cellulose acetate, butylacetate, xylene and ethyl cellosolve. The coating was at least partially etched with a YAG laser. The patentee discloses that the coating absorbs in the infrared range. The patentee suggests that a suitable beam may be applied by a YAG (yttrium-aluminum-garnet) laser which has an effective wavelength of about 1.06 xcexcm or by an argon laser beam which has an effective length in the range of from 0.48 to about 0.52 xcexcm. It is not clear whether all the coating was removed from the aluminum substrate although the text alludes to this result. The patentee discloses that the etched areas became sensitive to UV light, and that the etched areas, after exposure to UV light and development, accepted ink, while the areas which were not etched accepted water. No quantitative results are presented. There is no indication that the liquid coating in the reference would be usable as a flexographic printing plate. There is no indication that the laser ablation was precise enough to allow removal of a polymer layer to uncover a photosensitive polymer layer directly beneath.
Lasers have also been used to physically transfer small amounts of polymer from one layer of a multilayer article to another. U.S. Pat. No. 5,156,938 issued to Foley et al. Oct. 30, 1992, relates to a method of laser-induced ablative transfer imaging suitable for the production of masks (negatives) for the graphic arts and printed circuit industries. In this process, a laser-sensitive material is physically displaced from a donor layer of a multilayer structure to a receptor layer. The image radiation-ablative top coat comprise at least one sensitizer that absorbs at the wavelength of the desired laser output in the near infrared spectral region of 760 nm to 3,000 nm and at least one ablative binder. The sensitizer is present in an amount sufficient to effect the rapid partial decomposition of the at least one binder when the sensitizer interacts with laser light.
This is described as an ablative transfer because some of the materials from the donor layer are ablated while other materials are deposited on the receptor layer.
The inventors have discovered that if a slip film, of the type already in use with flexographic plates, is modified with a strong UV absorber, a laser can be used to engrave the slip film instead of the photopolymer. The slip film, then, effectively becomes a negative that is created in situ. There is no need to separately manufacture a negative, or to eventually dispose of silver halide. Also, the light scattering effects resulting from the presence of a separate conventional slip film underlying the negative are eliminated, thereby increasing resolution of the image.
It is therefore an object of the present invention to provide a method of making a printing plate which does not require the use of a photographic negative.
Another object of this invention is to make a laser-imageable printing plate.
Yet another object of this invention is to provide a UV absorbing layer for a photocurable article that can be conveniently and accurately removed by laser ablation from the article.
The objects of this invention can be accomplished by providing a UV absorbing and photoablatable layer for a photocurable article comprising
polymeric matrix and
a dopant having a high extinction coefficient in the range of 300-400 nm, the layer responding to a threshold dosage of radiation at a selected wavelength by photoablation of the polymeric matrix. The layer is applied to a photosensitive article, and then a laser is employed to remove, via ablation, selected areas of the absorbing layer, exposing the photocurable composition underneath to subsequent exposure to UV light and cure. The cured plate then can be developed in the normal fashion.
Other objects and advantages of this invention will become apparent through the disclosure herein.